It depends. Please ask your friend to check the app's "Deleted" folder -- and if they're there, he/she can select them and "move back" to the Inbox (the "up arrow" icon).
But -- with how POP3 works, those messages would have been in the Inbox on the mail server still.
So it's best to directly check the account's (the mail provider's) web mail / web site.
If the messages are *not* there, well, they're gone then.
